Underreamers are a type of borehole operating tool for use in enlarging the borehole in an oil well or mine, which borehole was initially bored by the drill bit. A typical underreamer includes expandable arms mounted in a housing by suitable hinge pins for movement between a withdrawn or closed position and an open, expanded position. Typically, the expandable arms are moved outwardly by means of a pressure actuated piston mounted within the main bore of the tool housing. In one type of underreamer, the expandable arms have mounted on the end, rotating cone bits for engaging certain types of formation and enlarging a borehole. Another type of underreamer is known as the "drag-type". In the drag-type underreamer, the expandable arms have a machined surface which is typically coated with a hardfacing material for engaging and expanding a borehole after the initial bore has been cut by a drill bit, or, such machined surface may have diamond bit implants such as manufactured by General Electric under the trademark "Stratapax". In oil well drilling application, these tools are mounted at the end of the drill string, except in the case of a drilling type underreamer, which is mounted in the drill string above the drill bit. One use of underreamers is to expand the size of the borehole in order to allow additional space for cementing operations or gravel packing. Another type of oil well drilling tool which utilizes expandable arms is a milling tool. In a milling tool, the arms are moved to an expanded position in order to engage casing, which is a steel tubing inserted into and used to encase the borehole, in order to cut the casing as needed. For the purposes of discussion here, these various tools, underreamers and milling tools, are referred to as borehole operating tools.
Such borehole operating tools used as underreamers engage the actual earth formation and cut a larger hole than created by the drill bit. In such a cutting operation, the cutter cones or hardened end of the expandable arms actually engage the formation and cut into the formation thus creating drill cuttings and formation pieces which need to be removed from the cutting area in order to make the cutting operation efficient. Cuttings from the formation need to be continually removed in order to keep the cutter element, whether it be a rotating cutter cone or a hardened outer end of the expandable arm, relatively free of debris and relatively clean and cool to enhance further cutting.
One means of keeping the cutter elements and the outer end of the expandable arms relatively free of debris is provided by providing nozzles in the tool body for spraying fluid outwardly at the cutter elements or the outer end of the expandable arms. As is well-known in the art of drilling oil wells, drilling fluid is circulated down through the drill string and returned upwardly in the annular area between the drill string and the wall of the borehole itself. Such fluid sprayed outwardly of the borehole operating tool body enhances circulation of cuttings made by the cutter elements of the tool arms upwardly in the annulus area. It has also been suggested and disclosed in certain patents that the expandable arms have one or more nozzles to spray fluid outwardly of the arms themselves; however, the problem of an effective fluid transfer from the underreamer body to the underreamers arms remains to be solved.